Apparatus and method for detecting surface defects on a workpiece such as a rolled/drawn metal bar

ABSTRACT

The present invention is directed to solving the problems associated with the detection of surface defects on metal bars as well as the problems associated with applying metal flat inspection systems to metal bars for non-destructive surface defects detection. A specially designed imaging system, which is comprised of a computing unit, line lights and high data rate line scan cameras, is developed for the aforementioned purpose. The target application is the metal bars (1) that have a circumference/cross-section-area ratio equal to or smaller than 4.25 when the cross section area is unity for the given shape, (2) whose cross-sections are round, oval, or in the shape of a polygon, and (3) are manufactured by mechanically cross-section reduction processes. The said metal can be steel, stainless steel, aluminum, copper, bronze, titanium, nickel, and so forth, and/or their alloys. The said metal bars can be at the temperature when they are being manufactured.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/430,549 filed Dec. 3, 2002, the disclosure of which is herebyincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made with United States government support underCooperative Agreement No. 70NANBOH3014 awarded by the National Instituteof Standards and Technology (NIST). The United States government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Related Field

The present invention relates generally to an imaging system that canimage the surface details of a workpiece, such as a rolled/drawn metalbar.

2. Description of the Related Art

It is known to produce a metal bar by a mechanical process such asrolling or drawing. Such metal bar is different than a metal slab,bloom, or strip (hereafter referenced as Metal Flat) in that the crosssection of such a bar has a smaller circumference/cross-section-arearatio such that the bar may rotate/twist about a longitudinal axis whilemoving forward longitudinally. For example, the bar shapes shown in FIG.2 have a ratio of circumference to cross-sectional-area that is equal toor smaller than 4.25 when the cross sectional area is unity for thegiven shape. The shape, when taken in cross section, of such a metal barmay be a round shape, an oval shape, or a polygonal shape, as shown as ahexagon, octagon or a square in FIG. 2. A metal bar of this type istypically referred to as “long products” rather than “flat products” inthe related industries. Rolling, drawing, extrusion and the like, asused in this disclosure and hereafter referenced as a Reducing Process,describe the ways for reducing the cross sectional dimensions of themetal workpiece through mechanical contact of the applicable tools, suchas rollers and drawing dies, and the workpiece. These Reducing Processesare generally continuous, or substantially continuous, in nature.

In the metal production industry, the presence or absence of surfacedefects is a relevant criterion upon which assessments of the metalproducts are made. For instance, surface defects account for half of theexternal rejects (i.e., rejected by the customer) for the steel bar androd industry. However, the conventional art provides no reliable meansto detect such defects. There are several challenges that conventionalinspection approaches have been unable to overcome.

First, in the case where inspection occurs while the metal bar productsare “hot,” the temperature can be as high as 1,100° C., preventing theuse of many inspection technologies. Second, the traveling speed of sucha metal bar as described above can be, presently, as fast as 100 m/s,several times faster than the speed of the fastest metal strip andnearly 100 times faster than a metal slab or bloom. Further, speedincreases are expected in the near future in the range of 150 m/s to 200m/s. Conventional inspection approaches simply cannot accommodate suchhigh traveling speeds. Third, a high temperature metal bar such asdescribed above is typically confined in a sectional conduit so that thebar will not cobble. Cobbling is an incident wherein a hot, high speedmetal bar runs freely outside the conduit. The space, therefore, for anyinspection device is extremely limited.

While it is known to apply various imaging approaches to the inspectionof cast or rolled Metal Flats, imaging technologies have heretofore notbeen used in Long Products (i.e., metal bar) inspection. Conventionalimaging systems are not believed capable for use in inspecting metalbars and the like because the geometry of the metal bars invalidate theillumination and imaging designs that are used to enhance/capturedefects on flat surfaces. FIG. 4 illustrates the differences of applyingillumination and of capturing images on a flat workpiece versus a roundworkpiece. As to the non-flat workpiece, the freedom in opticalalignment and optical working ranges disappears when the object ofinterest does not have a flat surface. For instance, the image line andthe illumination line may not overlap if the light or the camera istilted, as shown in exemplary fashion in FIG. 4. Furthermore, metal barstypically are at a higher temperature than Metal Flats. Heat dissipationof an object is proportional to the area exposed to the cooling media,such as ambient air or water spray. The area of a Metal Flat is severaltimes larger than that of a metal bar, assuming both the flat and thebar are made of the same material and both have the same longitudinalunit density and cross section area.

It is, however, known to employ imaging-based instruments for bar gaugemeasurement/control (shadow measurement), bar existence/presence, andbar traveling speed measurement in the Reducing Process.

It is also known to employ electromagnetic devices, such as eddycurrent-based instruments, in the assessment of long products.Eddy-current based sensing systems are used for the detection of surfaceimperfections in the Reducing Process for in-line inspection. Thisapproach has a high response rate, able to work in a high throughputproduction line environment (e.g., one kilometer of hot steel bars perminute). However, this approach has several drawbacks. First, it must bevery close to the hot surface (typically less than 2.5 mm). Accordingly,it is vibration sensitive and temperature sensitive. Moreover, it is notquantitative in the sense that it is NOT able to describe the nature ofthe detected defect. Finally, eddy-current approaches are incapable ofdetecting certain types of defects. As a result, the inspection outcomefrom eddy current devices is not used by the metal industry for adeterministic judgment on the quality of a specific product. Rather, theoutput of eddy current-based instruments is only used for qualitativeanalysis, such as “this batch of steel bars is generally worse than thebatch produced last week,” in the Reducing Process for process controlpurposes, for example, only.

Another approach attempted in the art employs ultrasonic sensing. Thisis an approach to replace the eddy current sensors with ultrasonic ones.However, many of the restrictions associated with eddy current-basedinstruments, such as the short working distance, apply with equal force.

Other inspection technologies used in the art include magneticpenetrant, circumflux, and infrared imaging with induction heating. Theuse of these technologies, however, is restricted. First, thesetechniques can only be used on “cold” metal bars. That is, thesetechnologies cannot be used for in-line inspection during or shortlyafter hot rolling applications. Also, the metal bars must be descaledbefore inspection. In addition, the use of magnetic penetrant is messyand cumbersome. This process typically relies on human observation withultra violet illumination, instead of automatic imaging and detection.The circumflux device is an eddy-current based unit, designed with arotating detection head. Such rotating mechanism limits the applicationof this device in metal bar inspection with high traveling speeds,typically used at about 3 m/s. Such device is also expensive due to themoving sensing head design. The combination of induction heating andinfrared imaging is based on the fact that induction current is onlyformed on the surface of the metal bar, and the surface defects on themetal bar will result in higher electrical resistance. Therefore, thespots with surface defects will heat up faster than other areas. Thereare issues associated with this approach in that (a) such faster heat upis a transient effect and thus timing (time to take images) is verycritical; and (b) infrared sensors are not available for very high datarates and therefore cannot support metal bars with high traveling speed.

Of course, inspection is possible after manufacture of the metal bars.However, post-manufacturing inspection often is not possible because theproduct is so long and coiled up, making the bar surfaces not accessiblefor cold inspection technologies.

Currently, real-time inspection of metal bars manufactured with ReducingProcesses is very limited. Metal bars are generally shipped from themanufacturer to the customer even if defective signals are posted by aconventional in-line eddy current inspection system. Customer complaintsmay therefore appear 3 to 6 months later due to surface defects on themetal bar products that are not immediately apparent to the customer.Such complaints cost the metal bar suppliers (i.e., manufacturers). Themetal bar suppliers will either refund the customers for the entirecoil/batch or cost share the expenses of additional labor to inspect theparts made out of the metal bar coil/batch.

There is therefore a need for an apparatus and method to minimize oreliminate one or more of the problems set forth above.

SUMMARY OF THE INVENTION

It is one object of the present invention to overcome one or more of theaforementioned problems associated with conventional approaches for animaging-based apparatus suitable to be used in-line or off-line todetect surface defects on rolled/drawn metal bars.

The present invention is directed to solving one or more of the problemsassociated with conventional metal bar inspection systems as well asproblems associated with applying metal flat inspection systems to metalbars for non-destructive inspection of surface defects on metal barsthrough the use of an imaging system.

One advantage of the present invention is that it may be effectivelyemployed in the production of metal bars with the aforementionedcharacteristics, namely, those that may be at a manufacturingtemperature, perhaps even hot enough to produce self-emitted radiation,as well as those subject to rotation relative to a longitudinal axis andmay potentially be traveling at a very high speed. Others advantages ofthe present invention include (i) effectively employed to image anddetect defects on non-flat surfaces; (ii) use for inspecting metal barsregardless of its temperature; (iii) use for inspecting metal barstraveling at speeds at or faster than 100 m/s; (iv) providing anincreased working distance to the metal bar surface, thus minimizing oreliminating the problems set forth in the Background for eddy-currentbased instruments; (v) providing an output comprising quantitative datawith verifiable defective site images; (vi) inspection of the workpieceeven before the scale forms on its surface; (vii) suitable for use ininspection at any stage (between the reducing stands or at the end ofthe line) in the reducing process, not affected by or relying upontransient effects; (viii) providing real-time or near real-time surfacequality information; (ix) providing a system absent of any movingsensing heads, thus minimizing or eliminating the problems of movingcomponents set forth in the Background; and (x) providing a systemneeding only very small gap (less than 50 mm) capable of operatingbetween metal bar guiding conduit sections. However, an apparatus and/ormethod need not have every one of the foregoing advantages, or even amajority of them. The invention being limited only by the appendedclaims.

A system is provided for imaging an elongate bar extending along alongitudinal axis. The system includes an image acquisition assembly, aline light assembly, and a computing unit. The image acquisitionassembly has a field of view configured to image a first predeterminedwidth over a circumference of a surface of the bar to define an imagebelt. The image acquisition assembly is further configured to produceimage data corresponding to the acquired image belt.

The line light assembly is configured to project a light line belthaving a second predetermined width onto the surface of the bar. Thelight line assembly is disposed, for example by alignment, relative tothe image acquisition assembly such that the image belt is within thelight line belt. The light line assembly is further configured such thata light intensity is substantially uniform along the image belt when thelight is collected by each of the image acquisition sensors.

Finally, the computing unit is coupled to the image acquisition assemblyand is configured to receive image data for a plurality of image beltsacquired by the image acquisition assembly as the bar moves along thelongitudinal axis. The computing unit is further configured to processthe image data to detect predetermined surface features of the bar. In apreferred embodiment, the detected features are surface defects and theimage acquisition assembly includes n digital cameras, where n is aninteger 3 or greater, arranged so that a combined field of view of thecameras corresponds to the image belt.

A method of imaging a metal bar is also presented.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, wherein like reference numeralsidentify identical components in the several figures, in which:

FIG. 1 is a schematic and block diagram view of an embodiment of thepresent invention.

FIG. 2 are cross-sectional views of exemplary geometries for work piecessuitable for inspection by an embodiment according to the presentinvention.

FIG. 3 illustrates a cross-sectional geometry of a metal flat.

FIG. 4 is a diagrammatic view illustrating a conventional lightingscheme as applied to a metal flat and a bar.

FIG. 5 are simplified perspective views illustrating a bar constrainedduring its travel by a conduit, and a gap between adjacent conduits inwhich an embodiment according to the invention may be situated.

FIG. 6 is a simplified plan view illustrating an imaging coverage on ametal bar using one camera.

FIG. 7 is a simplified plan view illustrating an imaging coverage on ametal bar with one camera and a telecentric lens.

FIG. 8 is a simplified plan view illustrating an arc length variationbased on a projection of same size grids, such as a line of pixels, ontoa bar profile.

FIG. 9 is a simplified plan view illustrating a lighting arrangement fora bar surface in accordance with the present invention.

FIG. 10 is a simplified plan view further illustrating, in greaterdetail, the lighting arrangement of FIG. 9.

FIG. 11 is a simplified perspective view of a metal bar in connectionwith which the lighting arrangement of the present invention is used.

FIG. 12 is a simplified plan view illustrating the lighting arrangementin the circumferential direction as directed toward a bar surface.

FIG. 13A illustrate a surface defect along with some surface noise.

FIG. 13B illustrates an exemplary result of an image processing stepaccording to the invention as applied to the image of FIG. 13A.

FIGS. 14A-14C illustrate examples of long surface defects that may befound on metal bars and which can be detected by an embodiment accordingto the present invention.

FIGS. 15A-15C illustrate relatively short surface defects that may befound on metal bars and which can be detected by an embodiment accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention permits automated inspection of metal bars forsurface defects as the metal bars are being rolled, drawn or the like(i.e., the reducing process described in the Background of theInvention). FIG. 1 schematically illustrates a preferred embodiment inaccordance with the present invention.

Before proceeding to a detailed description of the present inventionkeyed to the drawings, a general overview will be set forth. The presentinvention provides the following features:

-   -   1. Capable of working for metal bars manufactured through        reducing processes at different cross section geometry;    -   2. Capable of working for metal bars in-line at a bar        temperature up to 1,650° C.;    -   3. Capable of working for metal bars traveling at 100 m/s or        higher;    -   4. Capable of detecting surface defects whose critical        dimensions are as small as 0.025 mm;    -   5. Capable of reporting the defect nature such as its size,        location (on the bar), image, and the like;    -   6. Capable of accommodating different sizes of bars, for example        only, from 5 mm to 250 mm with minimum adjustment;    -   7. Capable of providing real-time or near real-time inspection        results;    -   8. Capable of working with a small access window (less than        50 mm) to the target object;    -   9. No moving parts while inspecting; and    -   10. Continuous operation in commercial, heavy industrial metal        production mills.

FIG. 1 is a simplified schematic and block diagram of a system inaccordance with the present invention. FIG. 1 shows line light assemblywhich may include at least one light source 2, a light conduit 4, aplurality of line lights 6 and a corresponding plurality of opticalboosters 8. FIG. 1 further shows a computing unit 10 and an imageacquisition assembly that may include a plurality of cameras 12 eachhaving a corresponding lens 14.

With continued reference to FIG. 1, a workpiece or object underinspection, such as an elongated metal bar 16 extending along alongitudinal axis, is shown moving along its longitudinal direction 20at a speed up to 100 m/s or faster while bar 16 is going through areducing process. The metal bar 16 may be formed from one selected fromthe group comprising steel, stainless steel, aluminum, titanium, nickel,copper, bronze, or any other metal, and/or their alloys. The bar 16 maybe solid or hollow. Typically such metal bar 16 is traveling inside aconduit, as shown in greater detail as conduit 24 in FIG. 5, not shownin FIG. 1. A gap 26, shown in FIG. 5, is defined between two adjacentconduits 24, and is typically very small, for example between about 20to 50 mm taken in the axial direction for high-speed transit of metalbars 16. It should be understood that metal bar 16 may be at an elevatedtemperature, for example as hot as 1,100° C. for a hot rolling process.It should also be appreciated that metal bar 16, given its geometry, isprone to twist/rotate about its longitudinal axis in the directionindicated by arrow 21 in FIG. 1 when it travels in direction 20. Thispossibility for rotation has, among other items, presented problems forconventional imaging systems. As will be described in greater detailbelow, the present invention overcomes this problem to provide animaging system that is robust to twisting and/or rotation.

In order to detect surface defects on bar 16, an imaging system inaccordance with the present invention must be provided having certainfeatures, as described below. With continued reference to FIG. 1, theimaging system includes an image acquisition assembly that preferablycomprises n imaging cameras 12, wherein n is an integer 3 or greater.The parameter n is selected to be three or higher based on an analysisset forth below. Each camera 12 is arranged so as to cover acircumferential span of at least 120° in order to image the entiresurface of bar 16. That is, the image acquisition assembly has acomposite or combined field of view configured to image the entirecircumference of the surface of the bar 16 to define an image belt 18.As further described below, the image acquisition assembly is furtherconfigured to produce image data based on the image belt 18. Theanalysis for the parameter n for the number of cameras will now be setforth.

As shown in FIG. 6, a regular lens 14 associated with camera 12 willhave a viewing angle (field of view) formed by the two tangential linesof sight 28 extending from a focal point of lens 14 to the surface ofbar 16. This viewing angle, when projected onto a non-flat surface, suchas the one shown in FIG. 6, will result in a circumferential coverage 30that is less than 180° and will be insufficient to cover 360° with onlytwo lens/camera units where the lens are not telecentric.

FIG. 7 shows an arrangement with a telecentric lens 14′. A truetelecentric lens, which collects lines of sight that are in parallel,even if used, would not practically provide for a two-lens/camera systembecause of arc length variation. In particular, the lines of sight 28are parallel with the addition of telecentric lens 14′ to lens 14. Inthis case, the circumferential coverage 30 is 360°. Theoreticallyspeaking, the entire surface of round shaped bar 16 can be covered usingonly two lens/camera units. However, as alluded to above, a problem ofnon-uniform pixel sizes arises.

As illustrated in FIG. 8, the evenly spaced lines of sight 34, asderived from an evenly-spaced imaging sensor 32 having a plurality ofpixels, can result in an uneven arc length 36 on the surface of bar 16,pixel-to-pixel. Even spacing is a very typical arrangement on an imagingsensor such as a CCD chip. The arc length 36 can be calculated usingequation (1) as follows:S=p/cos (θ)  EQN (1)where S is the arc length 36 mapped to the pixel in position y, p is thepitch of the pixel array or the pixel size, and θ is the projected anglethat can be derived fromθ=arcsin (y/r), in which y≦r and r is the radius of the metal bar16.  EQN (2)

From FIG. 8 one can learn that as y→r, θ→90°. As θ→90°, S the arc length36 will approach infinity based on EQN (1). In reality, S will still bea finite number. However, S will be substantially (several times) largerthan p, the pixel size. That is, the image resolution in this area willdeteriorate so much that this approach is infeasible. Note that the samearc length analysis can be applied to the bottom half in FIG. 8, inwhich case y→−r.

With three cameras, θ can be established at 60°. When θ=60°, S the arclength 36 (at the 12 o'clock and 6 o'clock positions in FIG. 8) is only2 p, an acceptable and controllable deterioration in image resolution.If better image resolution is desired, four cameras or five cameras, oreven more may be used (i.e., the parameter n referred to above can be aninteger equal to four, five or greater). All the lens 14/camera 12combinations, as illustrated in FIG. 1, are preferably arranged suchthat all such lens/camera combinations are positioned along a circularpath 22 that is concentric to the circular geometry of the exemplarymetal bar 16 such that the working distances, the distance from eachlens 14 to the nearest metal surface, are the same or nearly the samefor all the lens/camera combinations. Note that the path 22 may staycircular if the metal bar is non-circular, say a hexagon, for thepurpose of generally serving the same manufacturing line. One that isskilled in the art can appreciate that the path 22 can, if desired, bemade to conform to the actual bar geometry.

In order to accommodate the potentially very high traveling speed of themetal bar 16, high data rate cameras 12 are preferably used. The cameras12 in the system are thus preferably digital cameras, with digitaloutputs to computing unit 10. This digital output format is desirable toaccommodate the harsh environment for signal fidelity. This digitalformat image signal may be received by the computing unit 10 throughstandard communication channels such as IEEE-1394 (also known asFireWire), Camera Link or USB ports, or a special interface known as aframe grabber. Each camera 12 preferably is able to generate 10,000,000(or 10 Mega) pixels per second such that a defect feature that is 0.025mm×0.5 mm can be identified. It should be appreciated, however, that todetect larger features, a reduced resolution, and hence reduced datarate (in pixels per second) would be required. Line scan cameras arepreferred even though progressive (non-interlaced) area scan cameras canbe used when the bar 16 is not traveling fast. Line scan cameras have anadvantage over area scan cameras in that line scan cameras only requirea line of illumination, instead of an area of illumination. This willsimplify the illumination complexity caused by the non-flat surface. Inthe case of using line scans, all the cameras in FIG. 1 will be alignedsuch that their imaging lines will be forming a circumferential ring, animage belt 18, on bar 16. This alignment is necessary to address theissue of twist and/or rotation (item 21). If this alignment is not held,the twisting or rotating motion can result in incomplete coverage of thebar surface.

Referencing back to FIG. 1 again, each camera will have a lens 14 tocollect light reflected from the bar surface. Telecentric lenses (lensesthat collect parallel image light rays, as illustrated with FIG. 7) arepreferred for a more uniform arc length distribution, even thoughregular lenses can be used. In addition, cameras 12 may be configured toinclude a lens iris to control exposure, and further, preferablyconfigured (if included) with the use of a predetermined lens irissetting for improved depth of focus/field in the application.

With continued reference to FIG. 1, the imaging system according to thepresent invention also includes a line light assembly configured toproject a light line belt onto the surface of the metal bar 16.Preferably, the line light assembly includes a plurality of line lights6. These line lights 6 can be individual light sources, such as lasers,or light delivery devices, such as optical fiber lights, as shown inFIG. 1. The light delivery devices must work with at least one lightsource, as shown in FIG. 1. More than one light source can be used ifhigher light density is needed for illumination. For metal bars 16 thattravel at very high speed, the cameras may be light starved due to veryhigh line/frame rates equating to a relatively short exposure time. Anoptical booster 8 may therefore be used for each line light toconcentrate the light and increase the light intensity. This booster 8can be a cylindrical lens or a semi-cylindrical lens. To use the imagingsystem in accordance with the present invention for metal bars 16 thatare at an elevated temperature, the line lights and the boosters must bemade of special materials configured to withstand such elevatedtemperatures. Each line light 6, for example, may be configured to haveits own glass window to serve this purpose. In the case of optical fiberline light, the material that binds the fibers together must be able towithstand high temperature, such as the high temperature epoxy. Theboosters 8 must be made of materials that can withstand hightemperature, too. Usable materials include glass, Pyrex, crystal,sapphire, and the like.

FIG. 9 is a top view of the preferred embodiment shown in FIG. 1. Tocope with light starving, the alignment between the line lights and thecameras is important. As illustrated in FIG. 9, the surface of metal bar16 after the reducing process, before, for example, a descaling process,can be treated as a reflective surface. Therefore, the optical law setforth in equation (3) applies:“incident angle=reflective angle”  EQN (3)EQN (3) is preferably used in a preferred embodiment to maximize thereflected light that is captured by the plurality of cameras 12. Theline lights 6 will each emit the light ray 40, which is boosted by abooster 8 and projected onto the surface of the metal bar 16. The lightray 40 is reflected to the path 42 and received by the lens 14 andeventually by the camera 12. Note that in FIG. 9, the metal bar 16travels in the direction 20. The projected and reflected light rays 40and 42 form an angle 44, equally divided by the normal line to thesurface of the metal bar 16. This angle 44 must be as small as possible,due to the illumination problem described above that is associated witha non-flat surface, as illustrated in FIG. 4. In FIG. 4, the light line18′ and the image line 18 will not overlap on a non-flat surface. Theideal case is for the angle 44 in FIG. 9 to be 0°. As this is onlypossible by using a beam splitter, it is less practical to do so whenthe system is light starving due to inherent power losses imposed byusing a beam splitter for example. The highest efficiency a beamsplitter can achieve is 25%, assuming a 0% transmission loss. Therefore,the angle 44 is preferably selected so as to be reasonably small, suchas 1° or in its neighborhood. If necessary, a reflective mirror 38 canbe used to assist in packing the camera and the light for a small angle44. This is another reason to use line scan cameras in this application.Line scan cameras only need an image path 42 with a small width, such asfrom 5 to 30 microns. The angle 44 can be kept very small with thissmall image path feature.

FIG. 10 shows in greater detail a portion of the lighting setup of FIG.9. As mentioned above, the angle 44 will not be 0 degrees unless a beamsplitter is used. Therefore, each line light 6 must have a substantialwidth W (item 41 in FIG. 10). One can see that in FIG. 10 the metal bar16 has a centerline 46. The line 48 indicates the 60° mark on the barsurface, starting from the tangential boundary on the left hand side ofthe bar, as shown in FIG. 10, and increasing to the right. One cameramust be able to image the metal bar 16 for the upper half to this 60°mark line 48. In a three-camera embodiment, the above calculationsapply. If more cameras are used, the line 48 may represent 45° for afour-camera system, at 36° for a five-camera system, and so forth. Ifdesigned symmetrically, the camera can also image the bottom half of themetal bar 16 for 60°. In order to achieve this coverage, the light linewidth W must be greater than a threshold based on:W≧2·r·(1−cos 60°)·sin αEQN (4)in which r is the bar radius and a is the incident angle (half of theangle 44). The 60° can be replaced by another angle if a differentnumbers of cameras other than three are used in the inventive imagingsystem. This notion is further illustrated in FIG. 11, in which theimage line 42 is clearly curved differently, yet covered by the lightline 40. In other words, the image acquisition assembly (e.g., theplurality of cameras in the preferred embodiment) captures an image belt18 having a first predetermined width over the entire circumference ofthe surface of the bar 16. The line light assembly (e.g., the pluralityof line light sources in the preferred embodiment) projects a light linebelt onto the surface of the bar 16 having a second predetermined width.The line light assembly is disposed and aligned relative to the imageacquisition assembly such the image belt falls within the light linebelt. Through the foregoing, the problem of non-flat surfaces isovercome.

Additionally, these line lights must be positioned such that the lightintensity as reflected from a point on the bar surface to the camerathat covers that point is uniform for all the points on the image belt18 (FIG. 1). A more detailed illustration is shown in FIG. 12. All theillumination must follow the law described in EQN (3). FIG. 12illustrates this arrangement for one camera. It should be appreciatethat such arrangement may be duplicated for other cameras used in theinventive imaging system. Based on EQN (3), the angle formed by theincident light ray 40 and the reflected light ray 42 must be evenlydivided by the surface normal 50. As in FIG. 12, an illuminator 52preferably includes a curved surface. Illuminator 52 is a device whoseemitted light rays (perpendicular to this curved surface at the point ofemission) will be reflected by the surface of the bar 16 to the imagingsensor in camera 12 and lens 14 based on EQN (3). Note that curve 52need not be a circular curve. This curve 52 depends on the distancebetween the curve 52 and the surface of the bar 16 (i.e., target). Curve52 may not be a smooth curve if the bar is not circular. Even though anilluminator with curve 52 can be made with modem technologies, suchilluminator can only be used with bars 16 at the designated diameter. Insome applications it is not practical. An alternative is to simulatesuch illumination effect with an array of light lines 6 and 8, as shownin FIG. 12. Each combination of light line/booster can be madeadjustable such that its direction can be re-pointed as shown by item 54to accommodate targets with different diameters. The light line approachis also beneficial in the case that the bar 16 is not circular.

Referencing back to FIG. 1, a computing unit 10 is coupled to pluralityof cameras 12. The computing unit 10 is configured to receive the imagedata for a plurality of image belts 18 acquired successively by thecameras 12 as the bar 16 moves along the longitudinal axis in thedirection 20 (direction 20 best shown in FIG. 1). Frame grabbers may beused to receive the image signals. The cameras 12 in the system,however, are preferably digital cameras, as described above. Thecomputing unit may comprise one or more computers in order to haveenough computing power to process the image data. Image processinghardware may be used in conjunction with the software for fastercomputing speed. If multiple computers are used, these computers can belinked together through inter-computer links such as TCP/IP or the like.

In any event, computing unit 10 is configured to process the image datato detect predetermined features of the surface of bar 16. In apreferred embodiment, the features are surface defects. Thus, the imagedata will be processed for defects, such defects being shown inexemplary fashion in FIGS. 13A-13B. The images typically contain boththe real defects (e.g., item 302) and noise, such as scratch marks(e.g., item 304). Image processing algorithms, implemented in computercodes such as C, C++, machine languages, and the like, or implemented inhardware logic, are used to filter out the noise, and to detect the truedefects, as shown in 306. The defects to be identified can be long andhave a large aspect ratio, as shown in FIGS. 14A-14C, where item 308 maybe 1000 mm long, and item 310 may indicate a width of 0.050 mm. Or, thedefects can be short and have a nearly 1-to-1 aspect ratio, as shown inFIGS. 15A-15C. These algorithms are known in the art, but will bedescribed generally. A first layer of processing may involve acomparison of local contrast in the image, such as by comparing a firstpredetermined threshold to the local contrast. A second layer ofprocessing may involve applying a second predetermined threshold todetect the nature of the defect such as size, location, length and widthand the like.

The preferred embodiment described and illustrated in connection withFIG. 1 will also have protection against dust, water, vibrations, andother damaging factors in a typical metal process plant such as a hotrolling mill or a cold drawing mill.

Those skilled in the art shall appreciate the possibility of furtherrestrain the bar and separately using three or more single-camerasystems in the reducing process line for inspection.

Those skilled in the art shall also appreciate that covering (e.g.,inspection of) a portion of the bar surface less than the entirecircumference may be useful enough for statistical process controlpurpose in the reducing process line.

Those skilled in the art shall also understand that a very high speed(high data rate and high frame rate) area scan camera can be used inplace of the line scan cameras if only a certain portion of each of thearea scan images is used for processing.

One can also understand that if the metal bars are at an elevatedtemperature, an optical filter can be used in conjunction with the lenssuch that only certain wavelengths in the reflected light rays 42 (inFIG. 12) will be used to carry the surface information of the metalbars. Such wavelengths are those not emitted or not dominantly emittedby the metal bars at the said elevated temperature. For metal bars at orcolder than 1,650° C., the wavelength 436 nm can be used. In this case,an interference filter at 436 nm will be used with the lens. Thiswavelength can vary with the temperature. If the temperature decreases,longer wavelength can be used.

In a still further variation, the light line assembly may be configuredto include a strobe light, wherein the computing unit 10 synchronizesthe illumination (i.e., the strobing) with the image capture functionperformed by the image acquisition assembly (e.g., the cameras 12 in thepreferred embodiment).

In a yet still further embodiment, the computing unit 10 is configuredto maintain a running record of the detected defects, including (i) arespective location of each detected defect relative to a “start”position, such as the leading end, on the bar 16 being manufacturedthrough processes that mechanically reduce the cross-sectional area ofthe metal bars; (ii) a respective notation of the nature of the detecteddefect, such as the size, shape, contrast; and (iii) optionally, anactual image of the site of and surrounding the detected defect. Therecord may be useful to the supplier/manufacturer, for example, fordetermining an up-front discount, and may be provided to the customer(e.g., on a diskette or other electronic means) for use in furtherprocessing, for example, what portions of the coil to avoid or dofollow-up work on.

1. A system for imaging an elongated bar extending along a longitudinalaxis, said system comprising: an image acquisition assembly having afield of view configured to image a first predetermined width over acircumference of a surface of said bar to define an image belt andproduce image data corresponding thereto; a light line assemblyconfigured to project a light line belt having a second predeterminedwidth onto the surface of said bar, said light line assembly beingdisposed relative to said image acquisition assembly such that saidimage belt is within said light line belt, said light line assemblybeing further configured such that a light intensity is substantiallyuniform along said image belt; and a computing unit coupled to saidimage acquisition assembly configured to receive image data for aplurality of image belts acquired by said image acquisition assembly assaid bar moves along said longitudinal axis, said computing unit beingfurther configured to process said image data to detect predeterminedsurface features of said bar, wherein said image acquisition assemblyincludes n digital cameras, where n is an integer 3 or greater, arrangedso that a combined field of view thereof corresponds to said image belt,wherein said light line assembly includes a plurality of line lightsources each projecting light beams at a first predetermined anglerelative to a normal line from the surface of the bar onto which saidlight beams impinge, and wherein respective principal axes of saidcameras are disposed at a second predetermined angle relative to saidnormal line, said first and second predetermined angles being equal. 2.The system of claim 1 wherein said first and second predetermined anglesare about one degree.